Beta-voltaic devices have very high energy density and utilize radioisotopes as a fuel source. Radioisotope emissions originate from within a few microns of a radioactive material's surface at very low levels (nW/cm2-μW/cm2), despite the high power density in the bulk of the radioactive source (mW/cc-W/cc). Several semiconductor materials, such as Si, GaAs, GaP, GaN and diamond, may be used in betavoltaic devices.
However, silicon carbide (SiC) is the material used here for the production of beta voltaic devices, due to its wide bandgap. Moreover, in addition to its radiation hardness and ability not to degrade over time at higher temperatures and in harsh environments, SiC provides low leakage currents to effectively harvest low level emission rates from the isotope. The wide availability of high quality SiC substrates and epitaxy makes SiC the most practical of all semiconductors for betavoltaics, when performance and efficiency are considered. For example, Si provides 100 times less power conversion efficiencies than SiC. Thus, betavoltaic devices made from Si are suboptimal, due to this poor efficiency.
Semiconductor-based beta-voltaic batteries find applications in several areas such as security systems and medical implants (e.g. pacemakers). In order to increase power in these and other applications in the presence of low emission levels from radioisotopes, it is necessary to take advantage of the energy density of the device and develop device geometries and packaging which maximize the size and utilization of radioisotope surface area. This invention uses novel device configurations and packaging to maximize power in betavoltaic batteries and power output per unit volume.
Beta-Voltaic Devices
A SiC based beta voltaic radioisotope battery can produce several nanowatts (nWs) to milliwatts (mWs) of power, at 1 to 2 volts, with theoretical efficiencies in excess of 30% and measured efficiencies of 20%. Radioisotopes provide fuel for these devices and emit high energy electrons, or beta particles. The radioisotope tritium may be used. Other radioactive materials, such as Nickel-63, Phosphorus-33 and Promethium, may also be used. Utilization of beta emitters is attractive because of the short penetration distance of emitted electrons. For example, a high energy electron emitted from nickel-63 is effectively stopped by 25 microns of plastic or a layer of dead skin. Moreover, beta particles do not damage semiconductor materials and are easily shielded from sensitive electronics.
For several decades, electronics have become smaller and ubiquitous. In addition, power requirements for silicon-based electronics have been made low enough to enable the realization of nanowatt electronics, and asynchronous logic platform technologies are either projecting or exhibiting a performance of 24 pJ/instruction and 28 MIPS at 0.6V. Many medical applications can be powered with 1 to 10 microwatts (μWs) of average power. With such low power consumption requirements, a beta-voltaic battery source is able to continuously power the aforementioned electronics. Using these low power electronic elements, it is also possible to implement massive intelligent sensor networks which can monitor a large range of environments and infrastructures, or power a pacemaker or other implantable devices for over 25 years.
Silicon Carbide
SiC is a wide bandgap semiconductor, which is ideally suited for use in radioisotope batteries. The material's wide bandgap provides not only for radiation resistance in long term exposure to high energy electrons, but perhaps, more importantly, the shunt resistance of SiC diodes is high enough to allow efficient extraction of energy from a radioisotope source. Silicon (Si), the semiconductor industry workhorse, cannot realize sufficiently high open circuit voltages or power conversion efficiencies to be an optimal alternative for beta-voltaic batteries. Recent improvements in SiC substrate and epitaxial technology will enable the low dislocation and defect densities required for realization of beta-voltaic devices (including batteries) which utilize this material.
Theory of a Radiation Battery
The operation of a radiation cell is well-described by the solar cell equations. The main relationship is given byVoc=nVT ln(Jgen/Jss),  (1)
where Voc is the open circuit voltage, n the ideality factor, VT the thermal voltage=25.9 mV at T=300K, Jgen the current generated by the radioactive source, and Jss is the reverse saturation current of the diode used in the cell.
Using a tritium radiation source and SiC material, as illustrative examples, the current generated in the cell can be predicted as follows. The current generated in SiC by high energy electrons emitted from tritium is given as:Jgen=(Jβ*Emean β*(1−η))/Ee-h  (2)
where JGen is the net generated electron current, Jβ the net flux of beta electrons from the radiation source (˜3 nA/cm2 for tritiated water), Emean β the mean beta electron energy generated by tritium, which is 5.5 keV, Ee-h the mean electron-hole pair creation energy, which is 5 eV for SiC, and η which is the backscattering yield, which is (10%) for SiC.
It is worthwhile to mention that each high energy beta particle from tritium generates ˜1100× (5.5 keV/5 eV) current in the cell due to this e-h pair creation energy. The expected maximum current density in SiC is ˜2 μA/cm2. This assumes 100% carrier collection efficiency in the absorption region. For SiC, this absorption region is ˜0.5 μm. Such a predictive analysis can be carried out for any radiation source, such as Ni-63, Tritium, Phosphorus-33, Pm-147 or others (e.g. see the ref. MVS Chandrashekhar et al., Appl. Phys. Lett., 88, 033506 (2006)).
Radioisotopes
There are several candidate radioisotopes which can be inserted as a power source for beta-voltaic batteries. These radioisotopes include, but are not limited to Phosphorus-33, Ni-63, Promethium and Tritium. All of the sources share the following drawback. Although the Curie load is calculated from the total volume of the radioactive material, the amount of useable energy is limited to the number of high energy electrons which escape from the surface of the source before they can be reabsorbed. The self-absorption length of these radioisotopes is of the order of microns (ref. Everhart and Hoff, J. Appl. Phys, 42, 5837 (1971)). This means that the optimum thickness for the radioisotope source material (such as foil) is microns. Only electrons from a very thin layer of radioisotope source material are extracted. Therefore, to increase total power in a betavoltaic device, it is desirable to have greater radioisotope material and/or semiconductor surface area rather than greater radioisotope material volume.